Ok, in the game there is an economic system. This means that each and every player has a wallet (Their balance). You can type the /balance command in chat to see how much money you have. You can earn money by selling certain types of blocks to the marketplace, or by trading items with players for money.
You can use the /pay <playerToPay> <AmountToPayThatPlayer>
to send other players money from your balance. They can also pay you.
If you have money you can use it to buy other goods from players. The other player gives you what you want, and then you pay that player for the item. You can also buy a wide variety of items from the in-game marketplace (by typing /warp market in the chatbox to get to the market quickly) with your money.
The $5 donation to get 50,000 credits means that by donating $5, you will automatically receive 50,000 in-game virtual currency credits as the perk. Remember that you are not buying anything when you donate to opticraft. You are simply donating, and you get a perk of some sort as a result, but it is all handled as a donation, and there are no refunds. All donations are final once you click the pay now button to make the payment go through. Also this server has a plugin which will automatically apply any perk you donate for to your account. If you donate $5 for the 50,000 credits, then your minecraft guy will have 50000 credits added within seconds. Even if you are offline, you will simply get the perks applied when you log back in. Note the credits only apply to Opticraft. You will not have these credits when you login on another server that may have an economy.
